What is a peak-flow meter?

Dr Babak Ashrafi

Medically reviewed by

Dr Babak Ashrafi

Last reviewed: 06 Jun 2023

Peak-flow tests for asthma

A woman using a peak flow meter

At a glance: The peak-flow meter

A peak-flow meter is a portable hand-held plastic device that helps people living with asthma measure how quickly their lungs can expel (blow out) air. It works by measuring how fast you’re able to breathe out – the numbered scale on the side shows the force of your breath in litres per minute.

Doctors recommend peak-flow meters to people with asthma to help them track their asthma. Even if you don’t feel any discomfort, the peak-flow meter can be able to detect any narrowing in your airways before your asthma symptoms worsen. This means that you can take steps to avoid your asthma worsening, such as adjusting your medication.

A peak-flow meter is a type of lung function test that can help you identify what could be making your asthma worse, if your treatment is working well enough, and if you need to get emergency medical help.

The device can be used by children as young as 5 and adults and young people.

What is a peak-flow test?

The peak-flow test, also known as a peak expiratory flow test or PEF rate, shows how well your lungs are working by measuring how fast you breathe out. It helps both you and your doctor to monitor your asthma by recording how well the airways in your lungs are working.

A peak-flow test is usually done at your annual asthma review appointment as part of routine asthma-management checks. Your doctor may also recommend that you carry out regular peak-flow tests at home using a peak-flow meter, and keep a diary of the numbers.

Peak flow scores vary according to the age, height and gender of the patient. Monitoring your peak-flow yourself can help you identify if your symptoms are getting worse and alert you to take action, such as using your asthma inhaler or getting medical help.

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How do you use a peak-flow meter?

A peak-flow meter is a handheld plastic tube with a mouthpiece on one end, which you breathe into as hard as possible. The scale on the side measures how strong your out-breath is. This measurement is your ‘peak-flow’.

To use the device:

  • make sure you are comfortable, while standing if you can
  • set the marker on the side of the meter to zero
  • take as deep a breath as you can
  • hold the peak-flow meter to your mouth and put your lips around it
  • blow out into it as hard and fast as possible (do a few practice breaths – as though you are blowing out the candles on a birthday cake)
  • write down a note of the reading from the numbered scale
  • repeat the above steps 3 times
  • write down the highest reading (your doctor or asthma nurse may ask to see these readings)

If you have asthma, or if your doctor suspects that you have untreated asthma, they may recommend that you use a peak-flow meter to monitor the function of your lungs. They might ask you to record your daily peak-flow readings by writing them down for a couple of weeks.

Once you’ve recorded your results, you will usually need to return to your doctor or asthma nurse to show them your readings. They will use the results to monitor your asthma by following the pattern of your scores. Your lung function can then be measured against your best peak-flow score to see how you are doing and if you need an adjustment to your asthma medication or further medical help.

Where do you get a peak-flow meter from?

You can get a prescription from your doctor for a peak-flow meter, or you can buy one from a pharmacy or online.

What does it tell you?

A peak-flow meter measures the strength of the airflow from your lungs and can help you and your doctor or asthma nurse monitor your asthma. If your doctor suspects that you may have asthma, readings from a peak-flow meter can help diagnose it.

Good to know: Peak-flow meters are also used during asthma attacks to show how severe the attack is and check how well you respond to treatment.

For people with chronic asthma, peak-flow meter scores are used to check your progress and determine if any changes are needed in the treatment and management of the condition.

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Keeping track of regular peak-flow scores in a diary can help to identify if your lung function is getting worse, before you may have even noticed any symptoms such as coughing or wheezing. A drop in your peak-flow meter scores can indicate problems with your airways. Monitoring any changes in your peak-flow readings can also detect the warning signs and prevent asthma attacks before they start.

Medically reviewed by:
Dr Babak Ashrafi Clinical Lead for Service Expansion

Babak studied medicine at King’s College London and graduated in 2003, having also gained a bachelor’s degree in Physiology during his time there. He completed his general practice (GP) training in East London, where he worked for a number of years as a partner at a large inner-city GP practice. He completed the Royal College of GPs membership exam in 2007.

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Last reviewed: 06 Jun 2023

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